Impulse Response (CAL1) Routine update

The calibration activities include the delivery of Cyclic Monitoring Reports summarising the assessment of the SRAL altimeter instrument. The performance of the instrument is assessed by monitoring the instrument calibration modes CAL1, CAL2 and AUTOCAL, as well as its temperatures on-board. Each report covers a cyclic period of 27 days and the details of each Sentinel-3A orbital cycle are available.

The CAL1 time delay is the main range instrumental correction impacting the determination of the Sea Surface Height. The CAL2 waveforms are used to compensate the effect of the system transfer function distortions on the science echoes. The figures below display CAL1 time delay for Ku-Band and CAL2 waveforms over the last period and since June 2016 respectively.

CAL1 SAR Ku Time Delay Whole Mission Trend

CAL1 SAR Ku PTR Width Whole Mission Trend

Averaged CAL2 Ku and C waveforms over the last period

Calibration with transponder

The processing of the TRP data is done with SRAL L1A products. Passes with IPF-SR-1 Version 06.13 use
reprocessed L1A and L2 data (cycle 3 to 23). Passes from
cycle 24 to 28 are processed with IPF-SR-1 Version 06.11 as
the reprocessed data is not available.

The range bias results are of the order of millimetres. The datation bias is of the order of hundreds of microseconds.

The passes on cycles 13 and 21 (08/08/2017) have not been analysed because the transponder was not switched on due
to extreme climate conditions.

The below figures present the results from the TRP passes processing. The range bias is computed as measured minus
theoretical. The results show a positive measured range,
7.92 mm larger than expected (elevation 7.92 mm shorter
than expected), and a datation bias of -165.01
microseconds, both extracted from the minimisation of the
RMS between theoretical and measured series, and from
the stack misalignment estimation. They also show a ~0.84
mm stack noise.

The regression line in the first shows a drift of 3.18 mm/year, but it has a very low significance.